THIS INVENTION relates to the production of liquid and, optionally, gaseous products from gaseous reactants. In particular, it relates to a process for producing liquid and, optionally, gaseous products from gaseous reactants, and to an installation for producing liquid and, optionally, gaseous products from gaseous reactants.
Many reactions, such as the Fischer-Tropsch synthesis reaction are highly exothermic and the effective design of a heat removal system is essential to control the reaction for industrial applications. This is also the case for the Fischer-Tropsch slurry phase reaction. Typically, heat removal is effected by passing boiler water through cooling pipes submerged in a slurry bed within which the Fischer-Tropsch synthesis reaction takes place. The boiling water is pumped from a steam drum through the cooling pipes and the heated water is then returned to the steam drum where it flashes to form steam. The steam passes out of the steam drum via a pressure control valve to a steam header. Often, the amount of steam generated is in excess of total requirements, but not enough high pressure steam is produced.
In the prior art of which the applicant is aware, the heat removal rate is matched with the heat generation rate of the Fischer-Tropsch synthesis reaction by varying the pressure in the steam drum. As will be appreciated by those skilled in the art, pressure changes in the steam drum changes the boiling temperature of the water in the cooling system and hence it changes the temperature of the water and steam in the cooling pipes in contact with the slurry bed, and the heat removal rate.
A disadvantage of the prior art heat removal system is that a sudden increase in heat generation in the slurry bed may cause operating problems, since a sudden increase in heat generation may cause a sudden drop in pressure in the steam drum, which may result in cavitation of the pumps that deliver the boiler water to the cooling pipes. This may result in a failure of the cooling system, leading to overheating of the slurry bed and thus damaging the catalyst in the slurry bed.
It is an object of this invention to provide a process and installation for producing liquid and, optionally, gaseous products from gaseous reactants, in which the temperature control of the slurry bed is improved and which can provide more optimum steam production.
According to one aspect of the invention, there is provided a process for producing liquid and, optionally, gaseous products from gaseous reactants, which process includes                feeding, at a low level, gaseous reactants into a slurry bed of solid particles suspended in a suspension liquid;        allowing the gaseous reactants to react as they pass upwardly through the slurry bed, thereby to form liquid and, optionally, gaseous products;        withdrawing any gaseous product and unreacted gaseous reactants from a head space above the slurry bed;        withdrawing liquid product and/or slurry from the slurry bed to maintain the slurry bed at a desired level;        passing boiler water, as a first heat transfer fluid, in indirect heat exchange relationship through the slurry bed to remove heat from the slurry bed;        allowing the heated boiler water to flash and separate to form pressurised steam;        controlling the pressure of the steam to be substantially constant; and        passing a second heat transfer fluid in indirect heat exchange relationship through the slurry bed to remove heat from the slurry bed.        
The first heat transfer fluid, which is boiler water, may remove at least 50%, preferably at least 75%, of the total heat removed from the slurry bed by the first and second heat transfer fluids.
The average temperature of the second heat transfer fluid in indirect heat exchange relationship with the slurry bed may be lower than the average temperature of the boiler water in indirect heat exchange relationship with the slurry bed.
The pressure of the steam may be controlled at at least 14 bar(g), preferably at least 16 bar(g).
The process may include cooling the second heat transfer fluid and returning it for heat exchange duty to the slurry bed. In other words, the second heat transfer fluid may be cycled continuously through the slurry bed, in a substantially closed system.
The cooling of the second heat transfer fluid may be effected by means of indirect heat exchange with a cooling fluid, e.g. air.
The process may include controlling the temperature of the slurry bed by controlling an operating temperature of the second heat transfer fluid passing in indirect heat exchange relationship through the slurry bed.
The second heat transfer fluid may be water. The process may include pumping the water to a pressure sufficient substantially to prevent evaporation of the water to form steam at the operating temperature and pressure of the water. Thus, the water may be pumped to a pressure of at least 28 bar(g), preferably at least 34 bar(g), e.g. about 40 bar(g).
Instead, the process may include allowing steam to be formed by the second heat transfer fluid. In this case, the water may be pumped to a pressure of between about 2 bar(g) and about 12 bar(g), preferably between about 4 bar(g) and about 10 bar(g).
The process may include selectively increasing a heat transfer surface area between the second heat transfer fluid and the slurry bed, and decreasing a heat transfer surface area between the first heat transfer fluid and the slurry bed, in order to increase the total heat removal rate achieved by the first and second heat transfer fluids. Instead, or in addition, the process may include selectively decreasing a heat transfer surface area between the second heat transfer fluid and the slurry bed, and increasing a heat transfer surface area between the first heat transfer fluid and the slurry bed in order to decrease the total heat removal rate achieved by the first and second heat transfer fluids. This may be effected by switching heat transfer surface area in contact with the first heat transfer fluid and the slurry bed to be in contact with the second heat transfer fluid and the slurry bed, and/or vice versa.
The solid particles may be catalyst particles for catalysing the reaction of the gaseous reactants into the liquid product, and, when applicable, the gaseous product. The suspension liquid may be the liquid product, with the slurry bed being contained in a reaction zone of a slurry reactor or bubble column using a three-phase system comprising solid catalyst particles, liquid product, and gaseous reactants and, optionally, product.
The gaseous reactants may be capable of reacting catalytically in the slurry bed to form liquid hydrocarbon product and gaseous hydrocarbon product by means of Fischer-Tropsch synthesis, with the gaseous reactants being in the form of a synthesis gas stream comprising mainly carbon monoxide and hydrogen.
The catalyst may be an iron based Fischer-Tropsch catalyst or a cobalt based Fischer-Tropsch catalyst. Typically, the catalyst particles have a particle size range such that no catalyst particles are greater than 300 microns and less than 5% by mass of the catalyst particles are smaller than 22 microns.
The process may include allowing slurry to pass downwardly from a high level in the slurry bed to a lower level thereof, through at least one downcomer located in a first downcomer region of the slurry bed, as well as through at least one further downcomer located in a second downcomer region of the slurry bed, with the second downcomer region being spaced vertically with respect to the first downcomer region, thereby to redistribute solid particles within the slurry bed, as disclosed in International Application No. WO 99/03574, the specification of which is incorporated herein by reference.
According to another aspect of the invention, there is provided an installation for producing liquid and, optionally, gaseous products from gaseous reactants, the installation including                a reactor vessel having a slurry bed zone which, in use, will contain a slurry bed of solid particles suspended in a suspension liquid;        a gas inlet in the vessel at a low elevation within the slurry bed zone, for introducing gaseous reactants into the vessel;        a gas outlet in the vessel above the slurry bed zone, for withdrawing unreacted gaseous reactants and, when present, gaseous product from the vessel;        a liquid outlet in the vessel within the slurry bed zone, for withdrawing liquid product from the vessel;        a first, steam-producing, cooling arrangement for bringing boiler water in indirect heat exchange relationship with the slurry bed zone, the first cooling arrangement including pressure control means for providing steam from the first cooling arrangement at a substantially constant pressure; and        a second cooling arrangement for bringing a heat transfer fluid in indirect heat exchange relationship with the slurry bed zone.        
The first cooling arrangement may include a steam drum and a steam header. The pressure control means may be configured or configurable to control the pressure in the steam header at a preselected set point.
The second cooling arrangement may be a steam producing cooling arrangement for producing steam at a lower pressure than the first cooling arrangement. The second cooling arrangement may thus include a steam drum.
The second cooling arrangement may be a closed cooling circuit which comprises an indirect heat exchanger for cooling the heat transfer fluid by means of exchange of heat with a cooling medium. The indirect heat exchanger may be an air cooler for cooling the heat transfer fluid with air. When the second cooling arrangement is a steam producing cooling arrangement and is a closed cooling circuit, it may include a condensate collecting drum in flow communication with the indirect heat exchanger for collecting condensate from the indirect heat exchanger.
The installation may include temperature control means for controlling the temperature of the slurry bed, in use. The temperature control means may be configured to control the slurry bed temperature by controlling an operating temperature of the heat transfer fluid in the second cooling arrangement.
The first cooling arrangement and the second cooling arrangement may be in selective flow communication with each other, to allow at least a portion of the first cooling arrangement selectively to carry heat transfer fluid from the second cooling arrangement, in indirect heat exchange relationship with the slurry bed zone, and/or vice versa.
The first cooling arrangement may have a pressure rating high enough to require the use of schedule 40 piping and 300 lb flanges.
When the second cooling arrangement is not a steam producing cooling arrangement, it may have a pressure rating high enough to require the use of schedule 80 piping and 600 lb flanges.